Voltage commutated chopper pdf

No notes for slide. Current commutated chopper 1. What is meant by current commutation? At to Thyristor T1 is turned on by gate pulse g1. Capacitor C is charged to -Edc — As T2 is on, the capacitor inductor series circuit is shorted and so the capacitor is charged to negative voltage -Edc. Now the current flows opposite through the thyristor T1 - The capacitor, charged to negative Edc, pushes current ic opposite to T1. Here ic doesn't not flow through diode, because the forward voltage of diode is more than that of drop accross thyristor T1.

So the impedance offered by T1 is more than diode, and the current ic flows through diode. So the diode is turned off. The commutation period is complete. The T1 is turned on by gate pulse. And again T2 is turned on, capacitor charges to negative potential and cycle repeats. What are the advantages of current commutated chopper?

The oscillating current in commutation circuit is given by ……….. Circuit turn off time for main Thyristor ………………. So, …………… Numerical problems a For a current commutated chopper, peak commutating current is twice the maximum possible load current.

For a maximum load current of A, calculate i The values of the commutating inductor and capacitor. A current commutated chopper is fed from a dc source of a V. Is a load current of A is assumed constant during commutating process, then compute the following; i Turn off time of main thyristor ii Total commutation interval iii Turn off time of auxiliary thyristor SOLUTION: i Peak commutating current Turn off time of main thyristor Total views 17, On Slideshare 0.

From embeds 0. Number of embeds 8. Downloads Shares 0. Comments 0. Likes You just clipped your first slide! The chopper operation is divided into four modes for the sake of convenience and better understanding. During this mode, there are two current paths as shown in figure-2 below.

Load current I o , assumed constant, constitute one path and commutation current i c the other path. Load current i o flows through source V s , main thyristor T1 and load whereas the capacitor current i c flows through oscillatory circuit formed by C, T1, L and D.

The capacitor current first rises from zero to a maximum value when the voltage across the capacitor is zero. As i c decreases to zero, the voltage across C becomes equal to the source voltage with reverse polarity i. This voltage across the capacitor is held constant at -V s by diode D. The voltage across auxiliary thyristor TA is equal to the negative of voltage across capacitor.

Therefore, when voltage across capacitor is zero, the same becomes zero across TA. The capacitor charging current i c is zero. Only main thyristor T1 continues to conduct during this mode and hence the chopper output voltage is equal to source voltage V s. This simply means that chopper is ON during this mode. This mode is shown in figure Basically, in voltage commutation, a large conducting thyristor is turned off by the application of large reverse voltage. This reverse voltage across the conducting thyristor reduces the anode current and rapidly it will reach zero.

Also, the reverse voltage existing across the thyristor will keep it in the off state. Hence, the further flow of current through the circuit is prevented. Here thyristor T M is the denotation for the main thyristor while T A is for the auxiliary thyristor, C is the capacitor, D is the diode and the load in the circuit is a combination of resistor, inductor, and capacitor.

Here FD represents a freewheeling diode in order to protect the circuit against inductive load. It is to be noted here that for the operation of the chopper to take place, it is mandatory that the capacitor is charged up to the supply input with a similar polarity i. So, in order to charge the capacitor in the circuit there are two methods:. Once the capacitor is charged with supply input and given polarity the chopper can begin its operation. Some necessary assumptions relative to the chopper circuit are as follows:.

Consider the circuit representation shown above where the capacitor is already charged with the peak of the supply input. Initially, T M i. In this case, two currents flow through two sections of the circuit, one is the load current i o while the other is the commutation current i c. The current starting from V dc flows through T M then reaches load is the load current i o.

While the other current flows in the oscillatory circuit that is constituted by capacitor C, thyristor T M , inductor L, and diode D.